Method and device for controlling the proportion of crystals in a liquid-crystal mixture

ABSTRACT

The invention relates to a method for controlling the proportion of aluminum crystals in a mixture of molten aluminum alloy and aluminum crystals. The electrical resistance of the mixture is determined with the aid of a four-point measurement. With the aid of the measured electrical resistance, the proportion of crystals in the mixture can be kept constant within narrow margins. The invention also relates to a device for carrying out the method.

The invention relates to a method for controlling the proportion ofaluminum crystals in a mixture of molten aluminum alloy and aluminumcrystals.

A mixture of molten aluminum alloy and aluminum crystals is present, forexample, in the fractional crystallization of contaminated aluminum.Fractional crystallization is a known method for purifying acontaminated metal alloy and is described, inter alia, in U.S. Pat. No.3,840,364. One of the forms of fractional crystallization is suspensioncrystallization. In suspension crystallization, the contaminated metalalloy is cooled slowly from the melt. As solidification begins, verypure metal crystallizes out, and as cooling continues metal which isprogressively less pure crystallizes out. By separating the crystalsfrom the uncrystallized liquid in the mixture, metal crystals consistingof an alloy with a significantly higher purity than the original metalalloy are obtained. In addition, suspension crystallization can also beused for a more highly contaminated metal, in which case the impuremetal crystallizes out first.

One problem with carrying out this method is that the proportion ofcrystals in the mixture is difficult to control. However, this isnecessary in order to allow industrial application of the method. In thecase of suspension crystallization, measurement of the temperature ofthe mixture cannot be used or can scarcely be used to control theproportion of crystals. On the one hand, the temperature which has to bemeasured is high, approximately 700° C. for aluminum, making thetemperature difficult to measure with accuracy, and on the other handpure aluminum will crystallize at one fixed temperature and therefore,at this temperature, may be either completely liquid or completelycrystallized, as well as every possible state in between. Therefore,temperature measurement cannot be used to control the proportion ofcrystals in the mixture; in the event of excessive cooling, the mixturemay solidify completely, without this being controllable with the aid ofthe temperature measurement. If the aluminum is slightly contaminated,there is a temperature difference between the completely liquid stateand the completely crystallized state, but this temperature differenceis very small and depends on the impurities present, which are notusually known with accuracy. Therefore, measurement of the temperaturecannot be used in the industrial suspension crystallization ofcontaminated aluminum to keep the proportion of crystals in the mixtureconstant within a reasonable margin.

It is an object of the invention to provide a method with which theproportion of aluminum crystals in a mixture of molten aluminum alloyand aluminum crystals can be controlled during, for example, fractionalcrystallization.

Another object of the invention is to provide a method of this type,which can be used accurately and on an industrial scale.

Yet another object of the invention is to provide a method of this type,which is simple and reliable.

In addition, it is an object of the invention to provide a device withwhich the method can be carried out.

One or more of these objects are achieved with a method for controllingthe proportion of aluminum crystals in a mixture of molten aluminumalloy and aluminum crystals in which the electrical resistance of themixture is determined with the aid of a four-point measurement.

The electrical resistance measurement makes it possible to accuratelycontrol the proportion of crystals in the melt, even if the percentageof crystals is not known, since the crystals have a significantly lowerresistance than the melt (factor of 2.2). With this method according tothe invention, it is possible to keep the proportion of crystals in themelt constant with sufficient accuracy for industrial applicationwithout it being known precisely which impurities are present andwithout any knowledge of phase diagrams.

However, the resistance in molten aluminum is low and has to be measuredvery accurately. Therefore, the resistance is measured with the aid of afour-point measurement in which a current is passed through the mixturebetween two points and the voltage in the voltage field between the twocurrent-carrying point is measured between two separatevoltage-measuring points. The two voltage-measuring points generate anegligible current, so that measurement of contact resistance isavoided. The current intensity which has to be used between thecurrent-carrying points is at least 5 amperes in order to allow avoltage of a few tenths of a millivolt to be measured over a path of 40mm.

The four-point measurement can be carried out completely independentlyof any temperature measurement.

The mixture is preferably stirred continuously, in order to keep aconstant ratio between crystals and melt throughout the mixture and inorder to counteract as far as possible the growth of crystals on thewalls of the mixing vessel in which the measurement is carried out.

The method is preferably carried out during the fractionalcrystallization of contaminated aluminum. Accurate control of theproportion of crystals in the mixture is very important in thefractional crystallization of contaminated aluminum.

The method according to the invention can be carried out in thebatchwise fractional crystallization of aluminum in order to separatethe mixture as soon as a defined percentage of crystals is obtained.

However, the method is preferably used if the crystallization is carriedout as continuous crystallization. In the case of continuous fractionalcrystallization, molten contaminated aluminum is supplied continuously,and a mixture of crystals and molten aluminum alloy is dischargedcontinuously. For this purpose, the mixture has to be cooled to agreater or lesser extent, inter alia as a function of the temperature ofthe contaminated aluminum supplied. The cooling of the mixture then hasto be controlled by the four-point measurement.

The electrical resistance is preferably kept constant in continuouscrystallization. The proportion of crystals in the mixture discharged isthen also constant and can be set to an optimum value. To keep theresistance constant, by way of example the cooling and/or supply anddischarge of the aluminum can be regulated.

The proportion of aluminum crystals is preferably kept constant within amargin of plus or minus 4%, more preferably within a margin of plus orminus 2%. A margin of this nature means that in a subsequent process themixture can easily be separated into crystals and molten aluminum.

The method is preferably used on aluminum in which the contaminationconsists at least in part of Fe. Particularly in the case of AlFe, theeutectic melting point is just below the melting point of pure aluminum,so that accurate control of the process and therefore accuratemeasurement are required.

The invention also relates to a device for controlling the proportion ofaluminum crystals with the aid of the method as described above,comprising a vessel for holding the mixture, a means for regulating thetemperature of the contents of the vessel, and a four-point ohmmeterwith two electrodes for current to pass through and having twoelectrodes for measuring voltage, which electrodes are preferablysurrounded by a protective tube.

The method according to the invention is therefore carried out using aknown vessel with temperature control, in which a four-point measurementis carried out using a four-point ohmmeter configuration which is knownper se and in which, however, the current-carrying electrodes arepreferably at least partially surrounded by a protective tube. Using theprotective tube protects the metal electrode from being dissolved in themelt and also leads to the current being fed into and out of the melt atthe correct locations.

The protective tubes preferably consist of ceramic material, morepreferably Al₂O₃. Ceramic material is able to withstand molten aluminum,and Al₂O₃ is a readily available and relatively inexpensive material.

According to an advantageous embodiment, the current-carrying andvoltage-measuring electrodes in the protective tubes contain moltenaluminum during use. The use of molten electrodes means that there willbe no oxide layer between the electrodes and the melt, so that thecontact resistance is low.

According to an advantageous embodiment, the current-carrying electrodesare positioned at a distance from the walls of the vessel, which is atleast equal to half the distance between the current-carryingelectrodes. As a result, in the event of any growth of crystals on theinner wall of the vessel, the current will nevertheless pass almostcompletely through the mixture between the current-carrying electrodes,and will not pass or will scarcely pass through the crystals on thevessel wall.

It is preferable for each of the voltage-measuring electrodes to bepositioned at a distance of at least 5 mm from the associatedcurrent-carrying electrode. Any growth of crystals on thecurrent-carrying electrodes then has little or no effect on the voltagemeasurement via the voltage-measuring electrodes.

The invention will be explained on the basis of an exemplary embodimentand with reference to the appended drawing, in which:

FIG. 1 provides a highly diagrammatic illustration of an exemplaryembodiment of the device according to the invention.

FIG. 2 provides a highly diagrammatic illustration of the currentcircuit of the four-point measurement according to the invention.

FIG. 1 provides a highly diagrammatic illustration of an embodiment of adevice 1 for controlling the percentage of crystals in a mixture of amolten aluminum alloy and aluminum crystals during the fractionalcrystallization of contaminated aluminum. The device 1 comprises avessel 2 for holding the mixture 3, protective tubes 4, 5, 6 and 7 forelectrodes for carrying out a four-point measurement, and equipment forregulating the temperature of the mixture (not shown) and stirringequipment (not shown). Obviously, an inlet and outlet for the mixture,insulation materials and the like may be fitted to the vessel, as isknown to a person skilled in this field.

The protective tubes 4, 5, 6 and 7 consist of ceramic material and eachhave a current-carrying electrode, I₁ and I₂, or a voltage-measuringelectrode, U₁ and U₂. The current-carrying and voltage-measuringelectrodes are illustrated in FIG. 1 as solid electrodes made fromaluminum wire with a diameter of 2 mm, which project beyond the ceramicmaterial.

During use, the current will flow from current-carrying electrode I₁ tocurrent carrying electrode I₂. The voltage-measuring electrodes U₁ andU₂ are positioned between the current-carrying electrodes I₁ and I₂,since that is where the voltage difference is greatest. To ensure thatthe current will not or will scarcely flow via (crystals on) the wall ofthe vessel, the electrodes must be positioned at a distance from thewalls of the vessel, which is at least half the distance between thecurrent-carrying electrodes. The distance a between thevoltage-measuring electrodes is, for example, approximately 50 mm. Thedistance b between the current-carrying electrode and the associatedvoltage-measuring electrode is, for example, 5 mm.

The possibility of the electrodes being partially melted is not shown.In that case, the electrode which in the solid state projects out of theprotective tube will be at least partially melted at its end and willform a hollow meniscus in the corresponding protective tube. Since theelectrode is (partially) melted at its end, the contact resistancebetween the mixture 3 and the electrodes will be very low, andconsequently the four-point measurement will have a greater accuracy.

FIG. 2 provides a highly diagrammatic illustration of how the four-pointmeasurement is carried out. A current I from a current source issuccessively passed through a reference resistance R1 and through themelt. The melt is obviously also a resistance, denoted by R2. Thevoltage is measured across the two resistances R1 and R2. The current Ihas to be measured constantly with a very high level of accuracy. Theresistance R2 can then be determined on the basis of the measured valuesfor the voltage across R1 and R2 and the known value for the referenceresistance R1.

Obviously, calculation and control equipment will have to be present inorder to measure and process the current and voltages and, on the basisof this information, to control the temperature-regulating equipment.

The method according to the invention will now be described withreference to FIG. 1.

A slightly contaminated aluminum is introduced into the vessel 2 andheated until it has completely melted. The protective tubes 4, 5, 6 and7 with the current-carrying electrodes I₁ and I₂ and thevoltage-measuring electrodes U₁ and U₂ are placed into the moltenaluminum. It is preferable to wait until the ends of the electrodes have(partly) melted. Then, a current with an intensity of, for example, 10Ais passed through the melt, in accordance with the arrangement shown inFIG. 2. Then the melt is slowly cooled with the aid of thetemperature-regulating equipment. At a given moment, crystals willprecipitate in the melt, which crystals will be distributed as uniformlyas possible through the melt by the stirring equipment. The crystalshave a composition, which is purer than the melt. Since the crystalshave a lower resistance than the melt, the resistance of the mixturewill drop. In theory, the percentage of crystals present in the melt canbe determined on the basis of the voltage between the voltage-measuringelectrodes U₁ and U₂, which is measured by means of the four-pointmeasurement, and a known composition of the contaminated aluminum. Inpractice, it will be necessary to determine, on the basis of experimentsfor a defined arrangement of specific electrodes, what percentage ofcrystals is present at what voltage.

The above text provides a description of a batch process. In practice,the method will preferably be carried out as a continuous process, inwhich completely melted contaminated aluminum is supplied continuouslyand a mixture of (more intensively contaminated) molten aluminum andcrystals is discharged continuously. In this case, it is important,above all, to keep the resistance in the mixture constant by means ofcooling or heating, so that the percentage of crystals which isdischarged is constant. The percentage of crystals which is obtained canbe adjusted slowly in a continuous process.

1. A method comprises controlling the proportion of aluminum crystals ina mixture of molten aluminum alloy and aluminum crystals, by determiningat least the electrical resistance of the mixture with the aid of afour-point measurement.
 2. The method as claimed in claim 1, in whichthe mixture is stirred continuously.
 3. The method as claimed in claim1, which is carried out during fractional crystallization ofcontaminated aluminum.
 4. The method as claimed in claim 3, in which thecrystallization is carried out as continuous crystallization.
 5. Themethod as claimed in claim 4, in which the electrical resistance is keptsubstantially constant.
 6. The method as claimed in claim 1, in whichthe contamination comprises at least in part Fe.
 7. A method controllingthe proportion of aluminum crystal in a mixture of molten aluminum alloyand aluminum crystals, comprising: introducing the mixture into avessel; regulating the temperature of the contents of the vessel;passing a current through the mixture with two electrodes of afour-point ohmmeter; measuring a voltage with two electrodes of thefour-point ohmmeter to control the proportion of aluminum crystal in amixture of molten aluminum alloy and aluminum crystals.
 8. The method asclaimed in claim 7, in which the electrodes are each surrounded by aprotective tube and the protective tubes consist of ceramic material. 9.The method as claimed in claim 7, in which the current-carrying andvoltage-measuring electrodes in the protective tubes contain moltenaluminum during use.
 10. The method as claimed in claim 7, in which thecurrent-carrying electrodes are positioned at a distance from the wallsof the vessel, which is at least equal to half the distance between thecurrent-carrying electrodes.
 11. The method as claimed in claim 7, inwhich each of the voltage-measuring electrodes is positioned at adistance of at least 5 mm from the associated current-carryingelectrode.
 12. The method as claimed in claim 7, in which the electrodesare surrounded by a protective tube.
 13. The method as claimed in claim7, in which the electrodes are each surrounded by a protective tube andthe protective tubes consist of Al₂O³.
 14. The method as claimed inclaim 7, wherein prior to passing the current through the mixture, anend portion of each electrode is partially melted in the mixture.